The present invention relates to an apparatus for erasing and extracting image data from a particular region of an original document, in which the particular region is specified by an exclusive marking sheet, by reading the sheet prior to the document, storing data associated with the sheet in a memory, and processing an image carried on the document based on the particular region which is stored in the memory. More particularly, the present invention is concerned with such an apparatus which is applicable to a digital copier, a digital printer, a printer using a mimeograph and the like.
An apparatus of the kind described is disclosed in, for example, Japanese Laid-Open Patent Publication (Kokai) No. 61-13867/1986. The apparatus there disclosed reads an original document and an exclusive marking sheet at the same density and stores the resulting data in a memory. For example, assuming that the density at which a document is read is sixteen dots per millimeter, data associated with the marking sheet of format A4 amounts to 15.9 megabits, i.e., 16.times.16.times.210.times.297=15.9 megabits, resulting in the need for an extremely large memory capacity. To reduce the memory capacity required, this prior art apparatus compresses data before storing the data memory and decodes the data after outputting the data from the memory. Such a scheme, however, has various drawbacks in that compressing and decoding data cannot be implemented without resorting to complicated and expensive circuits. Further, since the data are compressed, considerable difficulty is experienced in identifying a particular region specified by a frame in distinction from the rest of the data identifying region located outside of the particular region. Further, the processing applied to data which have been decoded is time-consuming because the amount of data is necessarily increased.
On the other hand, the content of processing may be varied from one kind of document to another, as has been practiced with some prior art digital copiers of the type using such a marking sheet. For example, a different kind of image processing may be applied to each of a text document and a photograph or like graphic document, which includes halftone, for the purpose of enhancing the reproducibility of an output image. Hereinafter, the processing applied to a text document will be referred to as a text mode, and the processing applied to a graphic document as a graphic mode. While in the text mode, data having undergone MTF (Modulation Transfer Function) correction are transformed into two-level data, i.e., black-and-white data by discriminating such data with respect to a predetermined threshold value, whereas in the graphic mode data are transformed by reproducing halftone based on a dither method or the like. However, should the processing associated with the text mode be directly applied to the graphic mode or vice versa, the reproducibility would be lowered to a critical degree. For example, in the case that a solid black image or characters are reproduced by processing them by the dither method which is associated with the graphic mode, the resulting image becomes mere fragments which are discontinuous due to local omission of data. A person, therefore, has to select either one of the text and graphic modes depending upon the kind of a document to be used.
Since an exclusive marking sheet previously described is similar in condition to a text document, it naturally has to be processed in the text mode. However, there is a fear that the sheet is inadvertently processed in the graphic mode, bringing about the above-described occurrence which would lead to erroneous recognition of a marked region of a document.